This invention relates to a differential amplifier used as a first stage amplifier or the like of an amplifying device in which a current mirror circuit for increasing load impedance is employed.
Conventionally, this type of differential amplifier with a current mirror circuit is as shown in FIG. 1. 1 and 2 are field effect transistors (FET) with N channels comprising the differential amplifier. The gates are connected respectively to input terminals 3 and 4 and the sources are connected in common and then connected to terminal 6 through resistor 5. Terminal 6 may be connected to a constant current source. 7 and 8 are PNP-type transistors comprising the current mirror circuit. The emitters are connected to the power supply terminal 11 by way of resistors 9 and 10 respectively. The collectors are connected to the corresponding drains of FETs 1 and 2 and the bases are connected in common together with the collector of transistor 8. The connection point B of the collector of transistor 7 and the drain of FET 1 is connected to the base of PNP-type transistor 12 of the next stage. The emitter of this transistor 12 is connected to power supply terminal 11 by way of resistor 13 and the collector is connected to output terminal 14 as well as to the terminal 6 by way of resistor 15.
The conventional differential amplifier with a current mirror circuit is as described above. In such circuits, considerations are given to integration of the current mirror circuit with the next stage circuitry and other circuits into integrated circuitry (IC) while attaching the active elements (in this case FETs 1 and 2) on the outside of the differential amplifier to improve S/N and associated interchangeability, as discussed in the above-mentioned related application. In this case, the current flowing to the differential amplifier (current I.sub.1 flowing to resistor 5) must be made variable to obtain the optimum value according to the type of active elements used.
However, in the case of the conventional differential amplifier with a current mirror circuit, when the current I.sub.1 is made variable, the voltage drop at resistors 9 and 10 changes, and the voltage between A and B (A is one end of the power supply side of resistor 9) changes. When the voltage between A and B decreases to a value smaller than that of the voltage between C and D (C is one end of the power supply side of resistor 13, and D is the other end of the transistor side of resistor 13) plus approximately 0.6 V, transistor 12 becomes non-conductive, and the circuit of the next stage does not operate. On the other hand, the voltage between A and B sometimes increases. When the voltage between A and B increases, the working point of transistor 12 increases and the operation of transistor 12 becomes unstable. Thus, a differential amplifier with a current mirror circuit is greatly affected by resistors 9 and 10. Consequently more stringent specifications for resistors 9 and 10 are required. However, if resistors 9, 10 and 13 are converted into integrated circuitry, the specifications therefor become rather approximate.